With squidlike tentacles extending from their distinctive multichambered shells, the extinct marine predators known as ammonites were once among the most successful and diverse animals on Earth. Scientists have identified more than 10,000 species from fossils found nearly everywhere on the planet where oceans once existed, from the Great Plains of North America to the foothills of the Himalaya and the glaciers of Antarctica.

Ammonite is actually the colloquial term for ammonoids, a large and diverse group of creatures that arose during the Devonian period, which began about 416 million years ago. Ammonoids are related to other cephalopods—such as squid, octopuses, and cuttlefish—and they were early relatives of the modern nautilus. Meanwhile, true ammonites are a